NMEA 2000

NMEA 2000 (IEC 61162-3) is the network marine electronics speak: chartplotters, GPS receivers, wind and depth sensors, engine gateways, battery monitors. Under the hood it is J1939 with a marine dictionary, the same 29-bit CAN framing, the same PGN/source-address structure, the same ISO address claim. Three things differ: the message catalogue, the multi-frame transport (Fast Packet), and how fields are packed.

It's J1939 at the wire level

Every NMEA 2000 frame is a standard CAN 2.0B frame with a 29-bit extended identifier, and the identifier is split exactly the way J1939 does it:

The PGN (Parameter Group Number) identifies what the message is, e.g. 127250 Vessel Heading, 130306 Wind Data. sloppyCAN reuses its J1939 ID parser verbatim and just swaps in a marine PGN dictionary when you pick NMEA 2000 from the protocol dropdown in the J1939 / N2K tab.

Fast Packet

A single CAN frame carries at most 8 data bytes. J1939 moves longer messages with its Transport Protocol (TP.CM / TP.DT); NMEA 2000 does not. It uses Fast Packet, which packs the sequencing into the data field of ordinary frames of the same PGN. Whether a PGN is single-frame or Fast Packet is a fixed property of that PGN (it's in the dictionary).

Byte 0 of every Fast Packet frame is a control byte: the top 3 bits are a sequence counter (0–7, ties one logical message together) and the low 5 bits are a frame counter (the ordinal within the message).

Byte 1 is the total payload length; bytes 2–7 carry the first 6 payload bytes.

Each continuation carries 7 payload bytes, placed at offset 6 + (frameCounter − 1) × 7. Collect frames until you have LEN bytes, then decode the whole buffer as that PGN. A Fast Packet message can be up to 223 bytes.

Field encoding

NMEA 2000 fields are little-endian and frequently bit-packed: a field can be 2, 4 or 6 bits wide and start anywhere, not just on a byte boundary. (J1939's whole-byte SPN model can't express this, so the marine decoder works in bit offsets.) Each field has a resolution, an optional offset, and a unit; the all-ones value means data not available and the next value down means out of range.

Worked Example: 130306 Wind Data

Wind Data is a single-frame PGN. The 8 data bytes lay out as:

That's exactly what the PGN Monitor shows when you run Demo with the dropdown set to NMEA 2000. It streams heading, engine RPM, position, wind, and a Fast-Packet engine-dynamic message so you can watch reassembly happen with no hardware attached.

Worked Example: 127245 Rudder

Rudder is a single-frame PGN. sloppyCAN's own boat only ever has an applied position to report, never a distinct commanded one, so the Direction Order and Angle Order fields stay not available and only Position gets written:

That −21° is exactly the contract's rudder_actual (−60%) converted through nmea2000.js's own hard-over constant — 35° of full throw — the one percentage-to-degrees conversion this file declares, because PGN 127245 carries a real angle and the contract's rudder signals are percentages.

Worked Example: 127237 Heading/Track Control

A 21-byte Fast Packet message (so it arrives as three CAN frames — the shape from the Fast Packet section above). sloppyCAN's boat only ever fills two of its many fields: the Steering Mode bits (which of the pilot's own two states it is in) and the Heading-To-Steer, both of which double as the closest legal expression of the contract's nav_mode_actual / heading_target. Every other field — the four limit-exceeded flags, Turn Mode, both rudder-order fields, the track and cross-track limits — stays not available, because this boat runs heading-hold alone and has no route, no cross-track error, and no distinct rudder order to publish.

In STANDBY the same two fields read 0 (Heading Control off) and the boat's current heading — never a sentinel, because the contract wants a bus that engages the pilot with no course of its own to capture whatever heading the boat is already on rather than steer to an arbitrary bearing.

Worked Example: 130577 Direction Data

A 14-byte Fast Packet message. Like the Heading/Track Control message above, most of its fields (Data Mode, COG Reference, SID, COG, SOG, Heading, Speed Through Water) stay not available — each already has its own dedicated PGN above with its own source address, so repeating them here would be the same value published twice from two different "instruments." Only Set and Drift, the tidal-current pair with no PGN of their own elsewhere, get written:

What makes a marine dash different from a truck's

The same physical quantity has two names on a boat that a land vehicle never has to tell apart, because a boat's medium moves under it:

Depth has its own version of the same trap: 128267 reports water under the transducer, which rides wherever the hull floats it — not under the keel, and not to the sea bed from the surface. A shoaling reading and a squatting hull both move that number the same direction, and the PGN has no way to tell you which.